Synchronous Motors

Synchronous motors have definite advantages in some applications. They are the obvious choice to drive large, low-speed reciprocating compressors and similar equipment requiring motor speeds below 600 rpm. They are also useful on many large, high-speed drives. Typical applications of this type are geared, high-speed (above 3,600 rpm), centrifugal compressor drives of several thousand horsepower.

A rule of thumb that was used in the past for constant speed applications was to consider the selection of a synchronous motor where the application horsepower was larger than the speed. This, of course, was only an approximation and tended to favor the selection of a synchronous motor and would be considered too severe by current standards. However, the rule can aid in the selection of the motor type by giving some insight as to when the synchronous might be chosen. For example, applications of several hp per rpm often offer a distinct advantage of the synchronous over the induction motor. In fact, at the lowest speeds, larger sizes and highest hp/rpm ratios may be the only choice.

One interesting characteristic of synchronous motors is their ability to provide power factor correction for the electrical system. Standard synchronous motors are available rated either 100% or 80% leading power factor. At 80% power factor, 60% of the motor-rated kVA is available to be delivered to the system as reactive kVA for improving the system power factor. This leading reactive kVA increases as load decreases. At zero load, with rated field current, the available leading reactive kVA is approximately 80% of the motor rated kVA. The unity power factor machine does not provide any leading current at rated load. However, at reduced loads, with constant field current, the motor will operate at leading power factor. At zero load, the available leading kVA will be about 30% of the motor rated kVA,

Because of their larger size, 80% power factor motors cost 15-20% more than unity power factor motors, but the difference may be less costly than an equivalent bank of capacitors. An advantage of using synchronous motors for power factor correction is that the reactive kVA can be varied at will by field current adjustment. Synchronous motors, furthermore, generate more reactive kVA as voltage decreases (for moderate dips), and therefore tend to stabilize system voltage better than capacitors, since they supply less leading kVA when the voltage is decreased.

When higher than standard pull-out torque is required, 80% leading power factor motors should be considered. The easiest way to design for high pull-out is to provide additional flux, which effectively results in a larger machine and allows a leading power factor. The leading power factor motor may, therefore, be less expensive overall. However, leading power-factor motors are generally "stiffer" electrically, and may need to be evaluated against the higher current pulsations that will result from reciprocating compressors or other pulsating loads. Larger flywheels can normally be applied to compensate for this effect.

in addition to power factor considerations, synchronous motor efficiency is higher than similar induction motors. Efficiencies are shown in Table 7-1 for typical induction and unity power factor synchronous motors. Leading power factor synchronous motors have efficiencies approximately 0.5-1.0% lower.

Table 7-1 Full Load Efficiencies

600

1800

3600

Hp

RPM

RPM

RPM

250

91.0 93.4*

93.5

94.5

1,000

93.5 95.5*

95.4

9.52

5.0(H)

97.2

97.0

97.2*

97.4*

10,000

97.5

97 „4

97.6*

15,000

97.8 98.1*

_

*Synchronous Motors, 1.0 PF Source: Modified from [7] & [15]

*Synchronous Motors, 1.0 PF Source: Modified from [7] & [15]

Direct-connected exciters were once common for general purpose and large, high-speed synchronous motors. At low speeds (514 rpm and below), the direct-connected exciter is large and expensive. Motor gener ator sets and static (rectifier) exciters have been widely used for low-speed synchronous motors and when a number of motors are supplied from a single excitation bus.

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